X-ray imaging array detector and laser micro-milling method for fabricating array

ABSTRACT

An x-ray focal plane array (XFPA) detector is fabricated by a laser micro-milling method under strict process control conditions. The detector has an array of phosphor pixels bonded together with a light reflective adhesive filling the grooves between adjacent pixels. The phosphor array is bonded on top of a visible detector array, either directly or via a light guiding structure, such that each phosphor pixel is aligned with a corresponding visible detector pixel. The phosphor array is fabricated by moving a phosphor substrate relative to a laser beam of predetermined intensity at a controlled, constant velocity along a predetermined path defining a set of grooves between adjacent pixels so that a predetermined laser flux per unit area is applied to the phosphor material, and repeating the movement for a plurality of passes of the laser beam until the grooves are ablated to a desired depth.

The U.S. Government has rights in this invention pursuant to contractNAS 1-97057 awarded by the U.S. Department of Defense under the SmallBusiness Innovation Research (SBIR) Program.

This application is a divisional of prior application Ser. No.08/937,552 filed Sep. 25, 1997, now U.S. Pat. No. 5,956,382.

BACKGROUND OF THE INVENTION

The present invention is generally concerned with x-ray or radiographicimaging systems and methods, and is particularly concerned with a methodof fabricating a high performance X-ray focal plane array (XFPA)detector for integration in such a system, as well as an imaging systemand method employing the detector.

An ongoing goal of medical and other diagnostic sciences is thedevelopment of a low-cost, high quality, high resolution, real-timedigital imaging system for x-rays transmitted through an opaque target,such as the human body. Such a system has the capability of providingon-line, noninvasive, multi-organ radiographic imaging. Such real-timedigitized imaging eliminates the need for film, and the time required todevelop such film.

Unfortunately, directly digitized detection of an X-ray image is aproblem because silicon used in the pixels of visible focal planedigitizing arrays or visual matrix detectors, such as charge coupleddevices (CCDs), has low responsitivity and is also damaged by X-rays.

It is known to place a fluorescent or phosphorescent medium between theX-ray source and a visual matrix detector to convert the X-rays tovisible light. There are still problems in using a screen of suchmaterial in front of the visual matrix detector. For example, if thescreen is too thin, not enough of the X-rays will be absorbed and somewill reach and damage the silicon in the matrix detector. If the screenis too thick, the induced fluorescence visible light is radiated in alldirections and is also scattered, enlarging the area of the illuminatedpoint source on the detector, thus blurring the picture and reducing thespatial image resolution. In some cases, scattered light may escapewithout reaching the matrix detector at all. The fluorescent orphosphorescent material may also have non-uniform properties, degradingthe image quality and resolution. Some phosphorescent materials exhibit"after-glow", in other words they may continue to emit light even afterthe radiation source is no longer present. This may further degrade theimage quality.

U.S. Pat. No. 5,519,227 of Karellas describes a structured scintillationscreen which overcomes some of these problems. Regions of a transparentor semi-transparent scintillating substance are ablated to form an arrayof individual pixels. Each pixel is surrounded with an opticallyinactive material having a lower refractive index, so that the pixel ismade to function as an optical waveguide. This confines the x-rayinduced phosphorescence to the individual pixels and channels it to thedetector. This increases resolution and detection efficiency. The methodof fabrication is as follows: The substrate of phosphorescent oroptically active material is exposed to electromagnetic radiation, suchas a laser beam, so as to ablate the substrate in exposed regions toproduce a one or two dimensional array of pixels. A mask may be placedin contact with the substrate so that the desired regions are ablated bythe laser beam. Following laser processing to form the pixels, thepixels are surrounded by an optically inactive interstitial material soas to avoid optical leakage from each pixel. The pixel structure isattached via a substrate to a detector such as a CCD camera.

Other XFPA medical imagers have also been proposed, and have beenintroduced commercially in recent years, particularly for dentalexaminations. However, these imagers have, up to now, been veryexpensive and demonstrate marginal performance, due to the significantchallenges in developing of a high performance, two dimensional XFPAdetection matrix. One of the problems is that in order to replacehigh-resolution film radiography, the pixelated detector must have highuniformity and almost zero defects, with a resolution approaching 20lp/mm, for good performance. All current commercial XFPA systems havedemonstrated inferior imaging quality as compared with state-of-the-artcommercial X-ray films, due to lack of sufficient resolution and lowsignal/noise.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a new and improvedlaser micromilling method which is particularly useful for fabricatingan XFPA detector. It is a further object of the invention to provide anew and improved XFPA detector radiographic imaging system.

According to one aspect of the present invention, a method formicro-milling a substrate to a predetermined depth is provided, whichcomprises the steps of directing a laser beam at a predeterminedintensity at a surface of the substrate material, moving the substratematerial relative to the laser beam at a constant velocity along apredetermined path so as to remove the surface of the substrate materialalong the path by application of a predetermined uniform flux per unitarea, and repeating the relative movement of the substrate materialuntil the material has been ablated to the predetermined depth.

Preferably, the substrate material is an X-ray fluorescent materialselected from a group consisting of CdWO₄, Bi₄ Ge₃ O₁₂, YAG:Eu⁺³,YAG:Ce, CSI(TI), CSI(Na), CSI, NaI, CsF, CaF(Eu), LiI(Eu), and Gd₂ SiO₅Ce. In a preferred embodiment of the invention, the focus and intensityof the laser beam is varied as the depth of the ablated grooveincreases, in a manner to keep the energy introduced into the materialat a constant flux per unit area, or energy per unit time per unit area.This helps to ensure that a relatively smooth-sided groove is produced,and also that the adjacent material is not degraded or damaged byintroduction of too much energy. The flux per unit area to be applied,in other words the power passing into the surface, is preciselydetermined, dependent on the melting point, opacity, and other criticalproperties of the material so that just enough energy is applied toablate the material in the desired region without spreading outwardlyfrom that region and potentially degrading adjacent pixels. The power orflux per unit area is controlled by means of the selected constantrelative velocity, and the laser beam intensity at the laser focusingpoint. All of these may be adjusted to achieve the predetermined fluxper unit area.

Preferably, a series of grooves are micro-milled in the substratematerial in an x-y grid pattern to form a reticulated pixel structurewith kerfs or grooves separating the pixels. The grooves are then filledwith glue material. The glue or adhesive material is preferablysubstantially reflecting the visible light, so as to optically isolateeach pixel from adjacent pixels. The resultant pixelated substrate maybe attached to a visible pixelated detector, such as a CCD (chargecoupled device) detector or other visual matrix detector. In this way,x-rays incident on the pixel array of fluorescent or phosphorescentmaterial will be converted to light rays, and these will be transmittedalong each pixel to the underlying corresponding visible detectingpixel.

The laser micro-milling technique, in which the laser travels atconstant velocity along each groove numerous times with a constantand/or controlled intensity to ensure uniform flux application per unitarea gradually ablating the material to a greater and greater depthalong each groove, ensuring that the groove walls will be relativelysmooth and uniform. Control of the laser focus and intensity helps toensure smoothness of the walls. This technique enables crystals withhigh quality specular optical properties to be used in an XFPA detector,without decomposition of the crystal material due to laser ablation,since the laser beam is not directed continuously on any one position ofthe crystal for an extended time period. The smooth optical finishing ofthe pixel walls as a result of the careful control of the laserprocessing helps to prevent any escape of light out of the individualpixels for internal light sources. Such loss is also prevented by thereflective glue material filling the grooves or gaps between adjacentpixels. The walls or sides of the pixels may alternatively oradditionally be coated with a highly reflective metal layer prior tointroduction of the glue or the glue may be mixed with metal spheres. .

Preferably, the laser in the micro-milling process is controlled to beswitched on only when the relative movement between the laser and targetor substrate has reached a constant velocity. Additionally, the laserpreferably has a continuous pulsed output, and the first, large pulsewhen the laser is first switched on is "killed" or removed in aconventional manner, to ensure that only uniform intensity laser pulsesare impinged on the target. The relative velocity and pulse timing issuch that adjacent laser pulses overlap to form a continuous groovealong each desired line in the x and y direction.

In one embodiment of the invention, a metal layer is deposited on top ofthe substrate prior to formation of the pixels. The metal layer willthen be reticulated along with the substrate to form individual pixelseach with a thin metal layer on top. The metal layer will be transparentto X-rays while reflecting any scattered light back into the crystal andincreasing optical isolation of the individual pixels. Alternatively,the metal layer may be deposited on top of individual pixels after thereticulation process is completed.

The completed, pixelated array is then attached either directly or via afaceplate to a visible light pixelated detector aligning thecorresponding visible detector pixels and fluorescent pixels to maintaingood registration.

According to another aspect of the present invention, a detector forX-ray focal plane imaging is provided, which comprises an X-rayfluorescent substrate having a first surface and a second surface, thefirst surface having grooves defining an array of micro-milledlight-guiding pixels, a light reflective material substantially fillingthe grooves between adjacent pixels, and an array of visible lightdetecting pixels coupled to said second surface of the substrate forreceiving light emanating from the substrate pixels.

In a preferred embodiment, each X-ray fluorescent pixel has a lightreflective material coating deposited on the first surface. In this way,light is reflected back into the respective pixels rather than beinglost or scattering into adjacent crystals, reducing or eliminatingcross-talk.

In a preferred embodiment of the invention, the X-ray fluorescentmaterial is selected from the group consisting of CdWO₄, Bi₄ Ge₃ O₁₂,YAG:Eu⁺³, YAG:Ce, CSI(TI), CSI(Na), CSI, NaI, CsF, CaF(Eu), LiI(Eu), andGd₂ SiO₅ Ce. These materials were selected based on their lasermicro-milling performance and X-ray detection performance. All of thematerials listed above are found to fulfill a great part of the requiredcriteria for effective scintillator material, to be compatible with thelaser micro-milling technique used to manufacture the reticulated array.Other materials with equivalent properties may alternatively be used inother embodiments of the invention.

In one embodiment of the invention, each of the x-ray fluorescent pixelshas a layer of reflective material such as metal covering the firstsurface. This increases light collection efficiency within the pixel.X-rays are transmitted through the metal layer, but light generatedwithin each pixel will be reflected back into the pixel if it isdirected at the metal layer. The pixels are suitably bonded to thevisible light detector array. In another embodiment of the invention,the phosphor or X-ray fluorescent material is bonded to a light guidingstructure which in turn is bonded to the pixelated visible lightdetector. In this embodiment, the x-ray fluorescent material may beselected from the group consisting of Gd₂ O₂ S:Tb, Gd₂ O₂ S:Pr, Ce, F,ZnCdS:Ag; Y₂ O₂ F:Eu.

The method of this invention allows an XFPA detector to be fabricatedwith high aspect ratio grooves having high optical quality surfaces andlittle or no material degradation. This results in an XFPA detectorstructure producing high resolution electronic X-ray imaging withexcellent imaging quality.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood from the followingdetailed description of some preferred embodiments of the invention,taken in conjunction with the accompanying drawings, in which likereference numerals refer to like parts, and in which:

FIG. 1 is a schematic vertical cross-section through an X-ray focalplane array detector according to a first embodiment of the presentinvention;

FIG. 2 is a schematic vertical cross-section through an X-ray focalplane array detector according to a second embodiment of the presentinvention;

FIG. 3 is a schematic vertical cross-section through an x-ray focal planarray detector according to another embodiment of the invention;

FIG. 4 is a top plan view of a part of the detector of FIG. 1.;

FIG. 5 is a perspective schematic view of a radiographic imaging systemusing the detector of FIGS. 1, 2 or 3;

FIG. 6 is a schematic illustration of an exemplary picture formed by thesystem of FIG. 5;

FIG. 7 is a block diagram of the system of FIG. 5;

FIG. 8A is a schematic illustration of a method of making a reticulatedphosphor detector by laser micro-milling, and illustrates the spatialand temporal control of the laser;

FIG. 8B illustrates a series of grooves formed by the method of FIG. 8A;

FIG. 8C illustrates one possible technique for milling a groove;

FIG. 8D illustrates the path of the sample relative to the laser beamduring milling of a series of spaced, parallel grooves according toanother technique; and

FIGS. 9A to 9D are schematic illustrations of a series of steps in thelaser micro-milling method.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIGS. 1 and 4 of the drawings illustrate an X-ray focal plane arraydetector assembly 10 according to a first embodiment of the presentinvention. The assembly basically comprises an array of reticulatedpixels 26 preferably formed from a selected phosphorescent material ofhigh uniformity, and a visible light pixelated detector 16 coupled tothe lower end of the pixel array such that each of the pixels 26 isaligned with a respective pixel of the detector 16. The phosphorescentmaterial of pixels 26 gives off light when exposed to X-rays 24. Kerfsor grooves 15 are located between adjacent pixels 26, and the groovesare completely filled with a glue material 19 which is substantiallyreflective to visible light.

Since X-ray photoelectric absorption is dependent approximately to thefourth power of atomic number (Z⁴), the material for forming pixels 26is of a high atomic number, preferably greater than 30. The thickness ofthe crystal, or height of the reticulated pixels, is arranged to besufficient to absorb substantially all the X-rays involved. The materialof pixels 26 may be a single high quality crystal, a glass or a plasticwith embedded fluorescence centers, as discussed in more detail below.

The upper end of each pixel 26 is coated with a thin film 22 of metal orother light reflecting material. The layer is preferably of a metal oflow atomic number, preferably less than 15, so that only minimal X-rayabsorption will occur in layer 22. Suitable materials are beryllium andaluminum, for example. The outer surfaces of the pixels 26 on theoutermost sides of the array are preferably coated with a thin film 23of a light reflective metal of high atomic number. Alternatively, allsurfaces of the pixels may be coated with a layer of metal prior tofilling the grooves with adhesive 19. All sides of the pixels will becoated with a metal of high atomic number while the top surface of thepixels are coated with a low atomic number metal. The detector 16 has anupper layer 18 of insulating material, and is attached to the lower endof the pixel array via a layer 17 of index matching adhesive material.

As illustrated in FIG. 4, the array of pixels 26 is preferably a square,x-y array, and each pixel is of square or rectangular cross-section.However, it will be understood that the pixels may be of other shapes,such as cylindrical, triangular, or the like, as long as each pixel inthe array is of identical shape and dimensions to all other pixels forachieving uniform imaging. Mixed shapes and sized of pixels may be usedfor special purposes, for example to compensate for irregularities inthe underlying visible FPA (focal plane array) detector.

In this embodiment, incoming X-rays 24 are converted to light rays 25which are guided along the pixels 26 into the underlying detectorpixels. The height of the pixels 26 is arranged such that substantiallyall X-rays will be converted into light before reaching the detector 16,which is preferably of a silicon material and susceptible to damage byX-rays. The use of a reflecting medium both between adjacent pixels andat the upper end of each pixel, as well as on all outer or externalfaces of the array, will reduce or eliminate light loss by scatteringfrom the respective pixels. Each pixel serves as a light guide forchanneling the converted visible photons to the active areas of thecorresponding visible detector pixel. The pixels are isolated from eachother by the light reflective glue material filling the grooves betweenadjacent pixels, which will substantially eliminate scattering lossesand cross talk between pixels. The pixels are fabricated so as to havevery smooth walls of excellent optical quality, so-that the pixels willbe able to serve as highly effective light guides for the convertedlight. The design of the pixel array has the potential of increasingsubstantially the light collection efficiency, beyond the approximatedoubling collection obtained by introducing the upper reflective layer.This can be very significant in reducing the X-ray dose required formedical radiography, and increasing sensitivity and resolution.

FIG. 2 illustrates an X-ray focal plane array detector assembly 20according to a second embodiment of the invention. In this alternative,instead of reticulating an X-ray phosphorescent material, a good qualityoptical crystal has a thin layer 14 of high performance scintillationpowder bonded to one surface. The assembly is then reticulated to forman array of optical pixels 12 each with a thin layer 14 of highperformance scintillation powder bonded to the top of each pixel. Aprotective or humiseal layer 21 is bonded on top of the phosphor layer14 prior to reticulation. As in the previous embodiment, the entireassembly is reticulated to form grooves or kerfs which are completelyfilled with a reflective adhesive material 19, and the upper end of eachlayered pixel is coated with a thin layer 22 of a light reflectingmaterial, preferably a metal of low atomic number, such as beryllium oraluminum. The outer side surfaces of the outermost pixels in the arrayare also coated with a thin metal reflective layer 23 of high atomicnumber. Alternatively, the pixel surfaces may all be coated with a thinmetal layer prior to adding the adhesive. The embodiment of FIG. 2 isotherwise identical to that of FIG. 1, and like reference numerals havebeen used for like parts as appropriate.

In this embodiment, X-rays 24 impinging on the upper end of the detectorarray will be converted to visible light in the upper phosphorreticulated layer 14. Resultant visible light rays 25 will be channeledthrough the optical guides 12 into the corresponding pixels of thevisible FPA detector 16 attached to the lower end of the array. Pixels12 are manufactured from high quality single optical crystal of glass orplastic material. The optical crystal is preferably of a high atomicnumber material so as to block any residual X-rays from reaching thesensitive visible detector 16.

One advantage of the embodiment of FIG. 2 is that the scintillator forthe bonded powder layer 14 and the optical guiding glass can each beindividually optimized, combining the expertise in the phosphor industrywith the well established manufacturing of excellent optical crystals.This detector assembly may therefore be less expensive to manufacturethan that of FIG. 1. However, a disadvantage over the previousembodiment is that light scattering in the powder scintillator isunavoidable, and may result in a degradation in resolution.

Although in the illustrated embodiment, light generated in the phosphorlayer is guided by means of a reticulated optical crystal into theunderlying visible FPA detector, alternative light guiding structuresmay be used in other embodiments, such as fiber optic faceplates.

FIG. 3 illustrates a modified detector assembly 30 which is similar tothat of FIG. 2 but which is assembled differently. In the embodiment ofFIG. 3, as in the previous embodiment, a high quality optical crystal isreticulated to form light guiding pixels 12 which are bonded to anunderlying visible FPA detector 16 in the same way as in the previousembodiment, and like reference numerals have been used for like parts asappropriate. However, unlike the embodiment of FIG. 2, where a phosphorlayer 14 is bonded to the top of the optical crystal prior toreticulation, the layered phosphor in this embodiment is reticulatedseparately from the optical crystal, and the two parts are then bondedtogether via an index matching adhesive layer 33.

A thin layer 27 of phosphor powder is bonded to a metal layer 29, whichin turn is bonded to a substrate or supporting layer 28 of a low atomicnumber material which is transparent to X-rays. The other face of thephosphor layer is coated with a protective humiseal layer 31. Theresultant multi-layer assembly is then reticulated to form a grid ofpixels separated by grooves or kerfs, and the grooves or kerfs arefilled with a light reflective adhesive material 90. The grooves orkerfs may be coated with a thin layer of a low atomic number metal priorto filling with adhesive material 90. Similarly, the reticulated opticalcrystal has grooves filled with light reflective adhesive material 91.The grooves may also be coated with a thin layer of metal prior toadding the glue, although the metal in this case will be of a highatomic number. The grooves in the multi-layer phosphor assembly extenddown into, but not completely through, the substrate or support layer28. The reticulated optical crystal and reticulated, multi-layerphosphor are then bonded together with the humiseal layer 31 facingdownwardly, such that each pixel in the multi-layer phosphor is alignedwith a corresponding light guiding pixel 12, as indicated in FIG. 3. Thesubstrate layer 28 used to support the bonded phosphor powder duringreticulation thus becomes the uppermost layer of the detector array.

This assembly operates in substantially the same way as the embodimentof FIG. 2. Incoming X-rays 24 pass through the substrate layer 28, whichis substantially transparent to X-rays, and are converted to light 25 inthe thin phosphor layer 27. The resultant light will then be channeledthrough the optical guides to the corresponding pixels of visible FPAdetector 16. As in the previous embodiment, the optical guides are ofhigh atomic number material so as to block any residual X-rays. Thereflective coating on all sides of each of the pixels will reduce oreliminate light escaping out of the pixels and cross talk betweenadjacent pixels, thus providing improved sensitivity and resolution.

Instead of a thin metal reflective layer bonded on top of the pixels, ahigh reflection, multi-layer dielectric coating of low atomic numbermaterial may alternatively be used. Metal layers may also be depositedon the sides of the pixels prior to introduction of the glue or adhesiveinto the kerfs or grooves, for enhanced reflection, as noted above. Themetal layers on the sides of the pixels will be of high atomic number inthe embodiment of FIG. 1. In the embodiment of FIG. 3, the side metallayers will be of low atomic number in the reticulated phosphor, and ofhigh atomic number in the reticulated optical crystal or glass.

The selection of the phosphorescent or scintillation material for therectangular pixel array is critical. The pixel array is fabricated by alaser micro-milling process, described in more detail below inconnection with FIGS. 8 and 9, and it is therefore important that theselected material is compatible with a laser micro-milling process. Thepixels may be fabricated either from a bulk single crystal or piece of auniformly phosphorescent material or from a layer material. Thefollowing criteria are the preferred rules for selection used inselection of the x-ray phosphorescent material for the pixel array:

1. High x-ray capturing cross-section at the specified energies.

2. High fluorescence conversion efficiency.

3. Visible fluorescence in the spectral range of the highest silicon FPAsensitivity.

4. Fast fluorescence rise and decay time.

5. High optical transparency to the fluorescence light.

6. Absence of long term light persistence or trapping (>1 ms).

7. Well behaved conversion response function, preferred linear.

8. Macroscopic and microscopic high uniformity and high performanceintegrity.

9. Mechanical robustness.

10. No scattering centers in the bulk or on either surface.

11. Both parallel surfaces optically polished.

12. Stability to temperature and other medical environmental conditions.

13. Thermally matched with the silicon FPA detector (similar expansioncoefficients).

14. Commercial availability at low-cost.

15. Compatibility with the laser micro-milling process.

The materials were selected both by research and by experimentalevaluation of materials which appeared to have the potential of meetingthe criteria listed above. Once suitable candidates had been located,the candidates were tested for compatibility with the lasermicro-milling method or process of this invention.

In the embodiment of FIG. 1, the material of pixels 26 may be anintrinsic scintillator or an extrinsic scintillator. There are thereforethree possible alternative types of scintillator material, two of which(intrinsic and extrinsic scintillators) are uniform materials used inthe embodiment of FIG. 1, and one of which is used in a bonded powderstate in the layered configuration of FIG. 2 or FIG. 3. A disadvantageof the embodiment of FIG. 1 using a single, micro-milled scintillatorcrystal is its availability and price. However, it has advantages overthe layered arrangement of FIGS. 2 and 3, since it reduces scatteringand may permit total x-ray absorption over the long pixel, withoutjeopardizing resolution.

The following two examples were found to be particularly suitable andalso met those criteria listed above. In the following examples, Example1 is for a scintillator layer as in FIG. 2 or FIG. 3, and Example 2 anintrinsic scintillator for the embodiment of FIG. 1.

EXAMPLE 1

In this example, a layer of Gd₂ O₂ S:Tb: (4 μm particle size) with a 7.5mg/cm² coating weight was coated on a 1 mm thick x-ray absorbing glass.The resultant coated glass was then micro-machined to produce an arrayof pixels or light guides each having a coating layer of x-rayphosphorescent material at the upper end, as in the embodiment ofFIG. 1. The fluorescence emission wavelength spectrum peaks at 545 nmand the decay time is approximately 1 mS. The substrate glass density is4.8 g/cm³, and it has a Young's modulus of 62.7×10² N/m², and atemperature expansion coefficient of 81.8×10⁻⁷ /C. The transmission ofthe fluorescence light at 545 nm through the substrate is above 92%.

EXAMPLE 2

In this example, a single crystal of bismuth germanate (Bi₄ Ge₃ O₁₂) wasused to form an array of phosphorescent crystals in the embodiment ofFIG. 2. This material is a high atomic number cubic (Eulytine) crystalof high density, 7.13 g/cm³. The crystal is an excellent phosphor forX-rays with a slightly lower fluorescence light output than CdWO₄, andit emits at around 480 nm. The index of refraction is 2.15, it has avery fast decay time of around 300 nS, and a relatively low afterglow (7mS). Its thermal coefficient is 7×10⁻⁶ /C. However, it has quite a largetemperature dependence response in the temperature range of 0° to 60° C.This crystal is relatively hard (Young's modulus=10.56×10¹⁰ N/m²) and isnot hygroscopic. It also does not have any crystalline cleavages.

Both of the above materials fulfill a great part of the desired criterialisted above, and are currently commercially available. Other suitablecandidates which exhibit high efficiency x-ray fluorescence along withmost of the other criteria listed above, are CdWO₄ (cadmium tungstate),Y₃ AI₃ O₁₂ :Eu⁺³ (yttrium aluminum garnet doped with europium), YAG:Ce,CsI(TI), CsI(Na), CsI, NaI, CsF, CaF(Eu), LiI(Eu), and Gd₂ SiO₅ :Ce.Other potential candidates are LuTaO₄ :Tb and LuTaO₄ :Nb, which may bebonded to an optical crystal or glass substrate, and single crystals ofY₂ O₃, which are very robust. Other examples for the layered detector ofFIGS. 2 and 3 are Gd₂ O₂ S:Pr, Ce, F, ZnCdS:Ag, and Y₂ O₂ S:Eu.

Of the foregoing examples, it has been determined that bismuth germanateis particularly advantageous for an XFPA detector. Theoreticalcalculations indicate that an XFPA digital imaging system using areticulated bismuth germanate (Bi₄ Ge₃ O₂) phosphor for conversion ofx-rays into light rays may exceed the imaging performance of acorresponding current XFPA imaging system by almost an order ofmagnitude.

The dimensions of the XFPA detector apparatus 10,20,30 and particularlythe array of pixels in each of the above embodiments, is also criticalfor achieving the desired performance. It is desirable for the kerfs orgrooves to be as narrow as possible, in order to minimize the dead areaof the detector. The groove should be as narrow and as deep as possiblewithout damaging the adjacent pixel structure. The micro-milling methoddescribed below allows the grooves between pixels to be made relativelynarrow, and the groove or kerf width is preferably in the range from 4μm to 15 μm. The cross-sectional dimensions of each pixel are preferablyin the range from 25 μm×25 μm to 250 μm×250 μm, and the height of thepixel is preferably of the order of 1 mm. The cross-sectional dimensionsof the entire array are determined by the dimensions of the availablehigh quality visible light pixelated detector. One suitable detectorwhich is currently available is a CCD detector, available in sizes ofthe order of 1"×1", with 1000×1000 pixels where the pixels are 25μm×25μm. The scintillator thickness is preferably sufficient tocompletely stop the penetration of any X-rays of the energy and dosagetypically used in medical or other radiography applications. Theselected dimensions of the groove produce an aspect ratio (kerf heightdivided by kerf width) preferably in the range of 5:1 to 150:1.Additionally, the laser processing provides pixel walls which areoptically finished or polished to a smoothness sufficient tosubstantially reduce or prevent scattering.

The selection of the glue or material 19, 90, 91 filling the groove orkerf in each of the above embodiments is also critical to optimumperformance of the detector. The material must be reflective to light,so that the surfaces of the adhesive material bordering the laserpolished surfaces of the pixels act as reflective surfaces to reflectall light rays back into the respective pixels, increasing pixelcollection efficiency. Additionally, the material should benon-transmissive, so as to provide optical isolation to preventcross-talk between neighboring pixels. Preferably, a concentration ofhigh atomic number components is included in the glue, so as to preventx-rays from leaking into the underlying radiation sensitive siliconvisible detectors and their readout circuitry. Suitable such componentsare platinum, lead, or other materials with an atomic number greaterthan 30. The scintillator thickness is preferably of the order of 1 mm,and, together with the introduction of x-ray absorbing components in theglue between adjacent scintillator crystals, will significantly reducethe potential for radiation damage to the underlying visible detector oractive silicon devices. This will significantly improve the reliabilityand operation longevity of the electronic imaging XFPA detector overthose previously available in the field.

The adhesive filler material selected preferably has the followingproperties:

1. Low viscosity and high wetting.

2. Relatively high melting temperature.

3. Chemical compatibility with other components.

4. Atmospheric chemically inactive.

5. Curing time less than 1 hour, without high temperature annealing.

6. High adhesion and high Young's modulus.

7. Smooth and shiny final surfaces for high optical reflectivity, and noshrinkage during or after curing.

8. Light absorption to prevent any light penetration.

9. Material which absorbs x-rays of the desired energy and dosage.

10. No secondary luminescence under x-ray or other radiation.

11. Thermally matched with the other materials.

Based on the foregoing criteria, adhesives which are suitable for use asthe adhesive filler 19 in the grooves or kerfs 15 are Epoxy 301 andEpoxy 509F. Preferably the epoxy is mixed with high purity tungsten,platinum, or other commercially available fine metal powder of highatomic number, with particulate size of the metal particles preferablybeing less than 2 μm. It is critical that the metal particles have asize much less than the kerf width.

Neither of the above two adhesives is affected by moisture, and both arehighly resistant to the environment's chemical and physical changes.They also demonstrate low viscosity, low exotherm, and good handlingcharacteristics. Any suitable commercially available index matching gluematerial is selected for the index matching glue layer 16.

In the embodiment of FIGS. 2 and 3, selection of a good quality opticalcrystal glass or plastic for the light guiding pixels 12 is alsoimportant. The criteria for selection of the crystal material are:

1. Good optical quality material, preferably a single crystal or glassor plastic.

2. Macroscopic and microscopic high optical uniformity.

3. Maximum optical transparency to the fluorescence induced in layer 14or 27.

4. Absence of any fluorescent centers.

5. Absorbing substantially all X-rays of energies and doses used.

6. Preferably high atomic number material (Z>30).

7. No optical scattering centers in bulk or surfaces.

8. Ability to obtain good polishing surfaces.

9. Stability to temperature and other environmental conditions.

10.Thermal match with visible detector and phosphor layer (similarthermal expansion coefficients).

11. Commercially available at low prices.

12. Compatibility with the laser micro-milling process.

13. Mechanically robust.

Some suitable materials which fall within the above criteria are glassesmade of high atomic number materials, and single crystals of KNbO₃(potassium niobate) or PbZrO₃ (lead zirconate).

An x-ray or radiographic imaging system, for medical or other purposes,such as non-destructive testing, utilizing the XFPA,detector describedabove will now be described in more detail with reference to FIGS. 5 to7. Although a medical application is described below, it will beunderstood that the imaging system may alternatively be used forexamining and testing in other fields. As mentioned above, a single XFPApixelized detector as described above will typically be limited by thecurrently available detectors for the fluorescence induced light. If aCCD detector is used, the low cost detector matrix currently availablehas a size of the order of 1 inch by 1 inch. Thus, for visualizinglarger objects, a number of XFPA detectors may be butted side by side tocover the area of interest. However, this results in a rather bulkydetector for target objects of up to 20" by 20" in size, such as bonesand body organs. Therefore, FIGS. 5 to 7 illustrate a preferredembodiment of a portable XFPA electronic imaging system utilizing theXFPA detector described above, in which a series of images are taken ina "step and repeat" manner using one or a butted group of detectors, toeventually produce a complete image of the area to be scanned.

As illustrated schematically in FIG. 4, a stand for supporting an XFPAdetector 10 opposite an x-ray source 30 has a base 32 and a supportframe 34 adjustably mounted on the base for movement in perpendicular xand y directions via suitable drives 35,36, respectively, The supportframe 34 has a vertical bar 38 projecting upwardly from base plate 40which is movably mounted on base 32. A pair of parallel, spaced arms42,44 project outwardly from vertical bar 38 in a horizontal direction,forming a generally F-shaped structure. Although the stand is F-shapedin the illustrated embodiment, it will be understood that other shapesmay be used, such as a square or circular support, as long as thealigned x-ray source and XFPA detector are located and maintained inalignment opposite one another and can preferably be moved together in a"step and repeat" motion or other pattern to cover the object.

Although the x-ray source and XFPA detector in the illustratedembodiment are mounted opposite one another and in alignment in a rigid,common stand, they may alternatively be mounted separately andcontrolled by computer to move in the desired alignment.

In the illustrated embodiment, the x-ray source 30 is mounted adjacentthe end of the uppermost arm 42 so as to direct an *ray beam verticallydownwardly. The XFPA detector 10, 20 is mounted adjacent the end of thelower arm 44 so as to face upwardly towards, and in alignment with, thex-ray source 30.

An x-ray transparent support table 45 for an object 46 to be x-rayed,such as a hand as illustrated in FIG. 5 by way of example, is positionedbetween the arms 42 and 44. A series of small fiducials or spots 48 ofx-ray opaque material or ink are provided on the support table at spacedintervals in the x and y directions so as to produce a generallyrectangular array of spaced spots. Each set of four spots defines asquare or rectangular array of dimensions slightly less than the fieldof view of the XFPA detector 10 or 12. In other words, where thedetector 10 has dimensions of 1" by 1", the spacing between adjacentfiducials will be slightly less than 1".

The control system for the imaging system is illustrated schematicallyin FIG. 7. The x-ray source 30 and XFPA detector 10,20 are eachconnected to a power supply 49,50, respectively. The output of the XFPAdetector is connected to a data acquisition block 52, which is linked inturn to a data processing unit 54. The output of the data processingunit 54 is linked to a communication device 55, a storage unit 56, and adisplay unit 58 for real time display of the image of object 56. Acomputer 60 is linked to the x-y table 32 to operate a step and repeatcontrol 62. The motion system will step the support frame carrying thealigned x-ray source and detector preferably in a serpentine path whilethe object is held stationary on table 45, so as to raster an objectarea of 20" by 20", taking an image of the x-rays received by thealigned detector at every step position. It will be understood thatalternative movement patterns, other than serpentine, may be used ifdesired to cover the object area. Preferably, the motion is stopped forat least 100 mS while each image is taken. Computer 60 controls motionof the system, as well as the x-ray source shutter, the imagingsequence, the data acquisition, the communication and displaysubsystems.

The computer is also programmed to process and interface the data toproduce a final combined or mosaic radiographic image from the sequenceof successive images 64, as generally illustrated in FIG. 6. Theinterlacing of the images 64 is made possible by the location of theimages of fiducial spots 48 on each image. These locations are matchedby pattern recognition and data processing. This processing willrecognize the fiducial patterns and mathematically overlap theirprecise, spatial locations as best illustrated in FIG. 6, whichillustrates a mosaic pattern by way of example only. The double lines 65indicate the overlap between adjacent images in both the x and ydirection. Once the final image has been successfully combined, thefiducial marks will be removed from the image to produce a clearpicture, and the average data in adjacent pixels will be introduced toprovide image completion of the patterns inside. Although fiducials areused to create the image in this embodiment, it will be understood thatfiducials may not be required where the exact position of the source anddetector is known, and movement is precisely controlled.

The x-y motion table or base 32 must provide high precision movementwhich can easily maintain the required spatial accuracy and alignmentprecision along x and y axes. Some suitable motion systems which providethis degree of precision in an x-y motion table are linear servo motors,ball screws with servo motion, and ball screws with stepping motors. Thesupport stand for the x-ray source and detector must be sufficientlystiff to resist vibration, and must be a robust, light weight and highdamping material. A polymer composite material is preferred for thestand, since this has greater stiffness and dimensional stability thansteel, cast iron, or aluminum.

The selection of the x-ray source 30 is also critical in producing ahigh resolution image. A large area radiating x-ray source will resultin wide shadow imaging of a narrow object, thus causing blurring of theimage. Thus, a small size x-ray source such as an x-ray microfocussource is preferably used. Such sources are commercially available inthe 10 KeV to 100 KeV x-ray range, and can be considered as pointsources. Alternatively, a well collimated x-ray beam may be used.

The laser micro-milling method used for reticulating the phosphorsamples according to a preferred embodiment of the invention will now bedescribed in more detail with reference to FIGS. 8 and 9. The samemethod may be used for reticulating a single phosphor crystal as in FIG.1, or an optical crystal with a phosphor layer on top as in FIG. 2, orfor separate reticulation of a layered phosphor and an optical crystalas in FIG. 3, in each case with or without an upper metal layer. Themethod requires precise control in order to achieve the desired highaspect ratio and narrow kerf grooves without causing any damage to thepixels themselves. This is because the materials which are most suitablefor forming the phosphorescent pixels are also materials which areparticularly difficult to laser machine properly due to their relativelylow melting points or are chemically unstable at higher temperatures.Other materials cannot tolerate wide temperature differentials, and willcrack under such conditions. Thus, the laser cannot be held at one spoton the crystal for any length of time, or the material will decompose.It is extremely important with such materials that the laser beamirradiance, or energy/unit time/unit area, be kept substantiallyconstant during the entire abrasion process, and be such that thematerial does not melt into the pixel structure, decompose, or crack.

There are a number of different parameters which must be controlled inorder to keep the irradiance at the desired constant level. In themethod as illustrated in FIG. 8A, a laser 70 directs a laser beam 72 atan underlying crystal or substrate 74 held in a stage or sample holder76 on a movable table 78 which can be moved in perpendicular x and ydirections and also rotated in a θ direction by x-y and θ drive. Thesample holder is designed in a conventional manner so as to lock thesample in a precise position and alignment during laser processing. Thetable may be any suitable x-y table, such as the Anoride Table producedby Anorad Corporation of New York. This moving table has a built inx-y-θ drive as well as a velocity feedback control to ensure asubstantially constant velocity. Movement of the table is controlled bycomputer 80 according to selected program instructions. Additionally,the laser has an adjustable focus to provide a z-direction adjustmentrelative to the table, also under the control of computer 80.

In the illustrated embodiment, the laser beam is held stationary in thex-y direction during the micro-milling process, while the table is movedback and forth in a generally linear path, let us say in the xdirection, as illustrated in FIG. 8C. Then the table is moved to thenext line, and so on, so that the laser beam mills a plurality ofparallel grooves 82 in the top of the crystal, as illustrated in FIG.8B. The table is then moved in the other direction (say in the ydirection) so that a set of parallel grooves perpendicular to grooves 82can be milled in a similar manner, forming a reticulated x-y gridpattern. The program controlling both the laser actuation and movementof the table is designed such that the laser is not turned on until themoving table has reached a constant velocity, at the desired location,as illustrated in FIGS. 8A and 8B, which demonstrates the on-offsequencing of the laser.

As best illustrated in FIGS. 8A and 8C, the computer starts a scan atpoint 0, about 50 μm prior to the start of the micro-milling at point A.Thus, the laser is off as the table accelerates up to constant velocity.From A to B, the table velocity is constant and the laser is on. Atpoint B, the laser is turned off, while the table moves on from thispoint, starting to decelerate after point B to point E. At point E, thedirection of movement is reversed. The table then accelerates back inthe opposite direction and the laser is turned on again at point B, whenthe velocity is again constant and stopped at point A, after which thetable decelerates to point 0. The same line is then repeated along thesurface of the crystal.

The laser intensity and velocity of travel is such that only a smallportion of each groove is milled during each pass of the laser along thegroove, as best illustrated in FIGS. 9A to D. After several passes ofthe 15 laser along a particular groove 15, the groove will be ablated toa bottom end 84a. FIG. 9B shows a successive stage where the groove hasbeen ablated to a depth 84b. As illustrated in FIGS. 9C and 9D, as thegroove deepens, it will act as a wave guide for the laser beam,concentrating the beam at the current lower end of the groove 84c,84d,respectively. Typically, the ablation rate is only about 2 to 10 micronsfor each pass of the laser beam along the groove, and between 20 to 120passes of the laser are typically preferred in order to fully ablateeach groove.

The milling of the reticulated array may be accomplished in a number ofalternative ways. For example, each groove 15 may be fully ablated tothe desired depth before the table moves on to align the laser with thenext groove in the array. Alternatively, each groove may be milled to acertain percentage of the desired depth, say 20%, before the table moveson to align the laser beam with the next groove. After all grooves havebeen milled to 20%, say, of the desired depth, the procedure is repeateduntil all grooves are milled to the desired depth. In these twoalternatives, the table will move back and forth with the laser beamtraveling back and forth repeatedly along the same groove, as generallyindicated in FIG. 8C, before moving on to the next groove. In anotheralternative, the laser beam may make one pass along all grooves in thex-y direction, and then repeat the same sequence for a second pass, andso on, until all grooves are milled to the desired depth in the samepass of the laser. The second of the above alternatives is preferred. Ineach case, the table will travel at a constant velocity from point A topoint B or back from point B to point A, preferably to within ±2%, dueto the feedback velocity control built into the x-y table.

FIG. 8D illustrates another alternative where the table movescontinuously in a serpentine path, rather than slowing down andreversing direction after every pass along a groove, as in FIG. 8C. Themovement starts at point O, then traverses the serpentine path frompoint O to point F, with the laser activated at the start and turned offat the end of each groove, i.e., the laser is on from point A to point Bin the first pass, from point B to point A in the second pass, and so onto the end of the serpentine path.

During the laser micro-milling process, the focus of the laser isadjusted periodically by a certain amount (z-direction adjustment) tooptimize the coupling of the light into the groove. Typically, the focusis changed by about 3 microns after about 20 passes of the laser. Theprocedure is repeated so that the laser beam travels along each of thegrooves numerous times, until each groove is machined to the desireddepth. The laser fabricated walls will be smooth and will act as lightguides for further processing of each groove, concentrating the light onthe end of the groove which is currently being ablated. Each groove ispreferably milled to a depth so as to almost penetrate the crystal, butterminates just short of the bottom face of the crystal.

In addition to controlling the direction and velocity of the table, andthe focus of the laser beam, and actuating the laser only when the tablehas reached a constant velocity, the laser parameters are also strictlycontrolled by the computer to ensure uniform processing and reduce therisk of damage to the individual pixels. The laser is preferably acomputer controlled Nd:YAG single mode CW laser which is Q-switched toprovide continuous pulses of less than 100 nS in width. The Nd:YAG lasercan be operated in the infrared 1.06 μm fundamental spectral line, thesecond harmonic line at 0.53 μm, the third harmonic line at 0.35 μm, orthe fourth harmonic line at 0.26 μm. The laser is preferably optimizedto obtain a pure TEM₀₀ symmetrical mode at the sample site. This mode ofoperation will result in the smallest focusing beam spot size at thetarget. The frequency of the acoustic switch is preferably in the rangefrom 0.5 to 30 kHz, and the intensity is preferably in the range from0.1 mW to 5 W. The laser is also controlled so that the first, largerpulse after the laser is first switched on is "killed", by providing afirst pulse killer in the laser control circuitry or by an externallight switch. Such circuitry is conventional in Q-switched lasers. Thesubsequent laser pulses will preferably be of substantiallyequalμmultitude and width to around ±2%.

The laser intensity and the velocity of the table, as well as the focusof the laser beam, are all controlled so as to provide a substantiallyconstant flux per unit area, or energy per unit time per unit area,along the groove. The flux per unit area is selected based on melting ordecomposition point and opacity of the crystal to be milled, such thatonly the material in the groove itself is ablated and the energy appliedis such that the walls of the groove will not be damaged. The irradianceor flux per unit area at the processing location may be in the range of1 to 10⁵ Watts/cm², depending on the characteristics of the substratematerial. For micro-milling of Bi₄ Ge₃ O₁₂, it was found that anirradiance of 40 Watts/cm² produced optimum results. The optimum fluxper unit area, to ensure that just the groove area is ablated and thesurrounding pixel structure is not damaged, may be determinedexperimentally for each desired phosphor material. The material isexposed to a low intensity laser beam, and the intensity is graduallyincreased until the material just starts to ablate in the beam area,without spreading into adjacent regions. This is then used as the laserintensity parameter for that material. For example, in the multi-layeredconfiguration, where there is an upper metal layer, a higher flux perunit area is preferably used for the first pass of the laser, in orderto cut through the metal, and the flux is then preferably lowered to thepredetermined level for the underlying phosphor material. The tablevelocity is preferably in the range from 0.5 to 0.001 inch/sec.

It has been found that, by strictly controlling the laser parameters andthe constant velocity of the movement of the crystal relative to thelaser beam, excellent reticulation of a phosphor crystal as in FIG. 1,or a layered phosphor as in FIGS. 2 and 3, can be achieved. This processproduces high aspect ratio reticulation in phosphor materials with agroove width of less than 6 μm between adjacent pixels. The laserfabricated pixel walls have been demonstrated to be very smooth and ofhigh optical quality, as a direct result of the strict control of thelaser and motion parameters. Due to the smoothness of the laserfabricated walls and their optical quality, the partial micro-milledgroove will itself serve as a light guide for further laser processing,allowing transfer of self-trapped laser energy to the bottom of thegroove for further micro-milling. This permits grooves of a very highaspect ratio, up to 120:1, and narrow width, of less than 6 μm, to belaser machined in materials which are conventionally considered to bedifficult to machine with a laser.

The laser micro-milling process described above may be used toreticulate a single piece of phosphor material, either in the form of aphosphor crystal or a glass or plastic doped with scintillator elements.

The phosphor material may be reticulated alone or with a thin metal filmor layer deposited on its upper surface, as in FIG. 1. The metal layermay alternatively be applied after reticulation. The process may also beused to reticulate an array as in FIG. 2, where a layer ofphosphorescent material in the form of a bonded powder is coated on topof an x-ray absorbing glass substrate, either with or without a metallayer on top. In either case, excellent results were achieved by strictprocess control as described above.

The method of forming the thin reticulated phosphor layer on top of thereticulated optical glass in the embodiment of FIG. 3 is preferably intwo stages. Firstly, a granular phosphor layer bonded with epoxy or thelike is deposited on a supporting substrate 28, preferably with anintervening thin layer 29 of metal. A humiseal layer 31 is preferablydeposited on top of the phosphor layer for protection of the phosphorlayer. The layered phosphor is then reticulated in the manner describedabove, with the grooves extending through the entire thickness of thephosphor and partially into the substrate. The humiseal protective layer31 may be of any suitable protecting polymer or epoxy material which iscompatible with the laser micro-milling process and the fluorescenceeffects. This layer adds strength to the granular phosphor layer, andavoids loss of particulates as a result of laser micro-milling, ensuringthat the milled pixels retain structural integrity.

A suitable optical crystal of equivalent dimensions is then reticulatedor laser micro-milled separately to produce a pixel array of identicalshape and dimensions to the phosphor structure. After filling thegrooves in both the layered phosphor and the optical crystal with areflective adhesive material, the reticulated phosphor structure isbonded to the reticulated optical crystal with the pixels in alignment.The advantage of micro-milling these components separately is that thelaser can be optimized to the particular material being milled in eachcase, ensuring the formation of uniform, smooth sided pixels through theentire structure.

The thin, coated phosphor layer may alternatively be bonded to anothertype of light guiding structure, instead of the reticulated opticalcrystal, such as a fiber optic faceplate, provided that the lightchannel diameters are smaller than the reticulation, to maintain theresolution required.

After formation of the pixels, either in a single, thick phosphor or inthe separately reticulated thin phosphor and optical crystal layeredstructure, the structure is cleaned to remove laser debris. Preferably,the pixel array is cleaned by means of a conventional ultrasoniccleaning process. The power and pulse shape of the ultrasonic cleaninghead is adjusted to produce effective cleaning of the laser debriswithout damage to the reticulated structure.

Although the pixels are preferably of square cross-section in theembodiments described above, it will be understood that they may be ofother alternative shapes, as required by the detector arrayarchitecture. Thus, pixels of rectangular, triangular, cylindrical, orother shapes may be micro-milled in an equivalent manner, by suitablecontrol of the x-y table drive.

After formation and cleaning of the reticulated pixel structure, thestructure is preferably checked using an optical high magnificationmicroscope and a scanning electron microscope to ensure that thecrystals demonstrate no collateral damage as a result of either thelaser micro-milling or the cleaning process. The selected adhesive ispreferably introduced into the grooves according to the followingtechnique. It is important that the grooves are completely filled by theadhesive material. This is preferably achieved by introducing theadhesive along one side of the crystal into the grooves under microscopeobservation, and allowing the adhesive to fill all of the grooves bycapillary action. The adhesive is then cured for the required timeperiod.

When the reticulated thick or thin multi-layered structure is fullycured, it is attached to the visible detector preferably via a suitableindex matching glue layer 16, maintaining the respective phosphor pixelsin alignment with the corresponding visible silicon pixels.

Any suitable high-performance, visible FPA imaging detector may be usedfor the visible FPA detector attached to the reticulated phosphorstructure. The detector is preferably silicon based. Some suitablesilicon FPA detectors that can be matched 1:1 with the reticulatedphosphor described above which are commercially available are theThomson-CSF 1024×1024, the SITE 2048×2048, and the KODAK KAF-1000 with1024×1024 photosensitive crystals. For example, the KODAK KAF-1000 has apixel size of about 24.6×24.6. If this is provided in a 1"by 1"size, andthe pixelated phosphor structure has similar dimensions with 25×25micron pixels, a matrix of 10⁶ pixels is produced with a 1:1 matchbetween the phosphor pixels and the silicon pixels, resulting in aresolution theoretically approaching 20 lp/mm.

Due to the excellent surface quality of the laser micro-milled pixelwalls, the phosphor pixels serve as light-guides for the inducedfluorescent light. The reticulation and reflective glue will preventmost of the cross-talk between pixels so that, with a 1:1 match to theunderlying silicon visible detector, the resolution of the sandwicheddetector will be theoretically comparable to that of the underlyingsilicon visible detector.

The thickness of the phosphor converter array is preferably designedsuch that almost no leakage of x-rays through the array and into theunderlying electronic circuitry can occur for radiation at the energyand doses typically used in medical or non-medical radiographyapplications. Experimental testing has shown that, for the embodiment ofFIG. 1, a crystal of 1 mm. thickness showed no transmission for x-raysat 40 KeV. For higher energy x-rays, in the 70 KeV level, only CdWO₄ andBi₄ Ge₃ O₁₂ crystals can be used to provide sufficient protection to theunderlying silicon detectors and electronics.

The x-ray focal plane imaging detector of this invention, fabricated byan innovative laser micro-milling method as described above, has thecapability of providing real-time, high performance radiographic imagingat relatively low cost. This will allow a small, portable X-ray imagingsystem to be carried by emergency personnel for field use, for exampleat accident scenes or on battlefields, permitting enhanced, immediatediagnostic evaluation. Such a system is also advantageous in the dentalfield, in order to permit real-time evaluations and adjustments, inaddition to cost savings and reduced radiation exposure. The "step andrepeat" method for taking a series of successive pictures scanned acrossan entire image will result in a lower x-ray total accumulated exposurethan with current exposure for x-ray film.

The controlled laser micro-milling method described above has thecapability of milling high aspect ratio, narrow kerfs or grooves inhighly x-ray sensitive, short life phosphor crystals which werepreviously considered unsuitable for accurate laser processing. Becauseof the strict control of the energy per unit area per unit time inputalong each groove in the crystal, it can be ensured that only theminimumμmount of energy required to ablate the desired region is putinto the crystal, reducing the risk of degradation of the groove wallsdefining the pixel array. This control is achieved by the constantrelative velocity motion of the sample being machined relative to thelaser beam, and by control of the laser intensity and pulse width. Sincethe energy per unit area per unit time is kept relatively adequate,numerous successive passes of the laser along the groove are required inorder to micro-machine the entire groove to the desired depth. Thehighly controlled laser processing results in fabrication of highlysmooth pixel walls of high optical quality. Thus, the fabricated pixelsserve as light guides for the x-ray induced light, with little or nocross talk between adjacent pixels or escape of light out of each pixeldue to the smooth optical finish of the side walls and the reflectiveand adhesive material filling the grooves.

Although some preferred embodiments of the present invention have beendescribed above by way of example only, it will be understood by thoseskilled in the field that modifications may be made to the disclosedembodiments without departing from the scope of the present invention,which is defined by the appended claims.

I claim:
 1. A method for micro-milling a substrate to a predetermineddepth, comprising the steps of:directing a laser beam at a predeterminedintensity towards a first surface of a substrate to be milled; movingthe substrate material relative to the laser beam at a predeterminedconstant velocity along a predetermined path so as to ablate the surfaceof the substrate material along the path by application of apredetermined uniform flux per unit area such that a predetermined depthof material is ablated for one pass of the laser beam along the path;and repeating the relative movement of the substrate material and laserbeam along said path for a predetermined number of passes until thematerial has been ablated sufficiently to form a groove of predetermineddepth and width, the ablation rate being between 2 to 10 microns foreach pass of the laser beam along the groove.
 2. The method as claimedin claim 1, wherein the substrate material is an X-ray fluorescentmaterial selected from a group consisting of Gd₂ O₂ S:Tb, CdWO₄, Bi₄ Ge₃O₁₂, YAG:Eu⁺³, YAG:Ce, CsI(TI), Csl(Na), Csl, NaI, CsF, CaF(Eu),LiI(Eu), Gd₂ SiO₅ :Ce, LuTaO₄ :Tb, LuTaO₄ :Nb, and Y₂ O₃.
 3. The methodas claimed in claim 2, wherein the substrate material is Bi₄ Ge₃ O₁₂. 4.The method as claimed in claim 1, wherein the flux per unit area is inthe range from 1 to 10⁵ Watts/cm².
 5. The method as claimed in claim 1,wherein the substrate material is Bi₄ Ge₃ O₁₂ and the flux per unit areais around 40 Watts/cm².
 6. A method for micro-milling a substrate to apredetermined depth, comprising the steps of:directing a laser beam at apredetermined intensity towards a first surface of a substrate to bemicro-milled; moving the substrate material relative to the laser beamat a predetermined constant velocity along a predetermined path so as toablate the surface of the substrate material along the path byapplication of a predetermined flux per unit area; repeating therelative movement of the substrate material and laser beam along saidpath for a number of passes until the material has been ablatedsufficiently to form a groove of predetermined depth; and adjusting thefocus of the laser at periodic intervals after a predetermined number ofpasses of the laser beam along the groove less than the number of passesnecessary to ablate the groove to the predetermined depth, whereby thelight is concentrated on an inner end of the groove which is currentlybeing ablated.
 7. The method as claimed in claim 1, wherein the laserbeam is directed along a predetermined set of spaced lines inperpendicular x and y directions to ablate a series of grooves in apredetermined x-y grid pattern and form a series of pixels with kerfs orgrooves separating the pixels.
 8. The method as claimed in claim 7,including the step of filling the grooves with glue material.
 9. Themethod as claimed in claim 8, including the step of depositing a thinmetal layer on the sides of each pixel prior to filling the grooves withglue material.
 10. The method as claimed in claim 7, wherein therelative movement between the laser beam and substrate follows a pathalong each of the grooves, and then follows the same path repeatedly fora predetermined number of passes until the grooves are ablated to thepredetermined depth.
 11. A method for micro-milling a substrate to apredetermined depth, comprising the steps of:directing a laser beam at apredetermined intensity towards a first surface of a substrate to bemicro-milled; moving the substrate material relative to the laser beamat a predetermined constant velocity along a series of parallel paths soas to ablate the surface of the substrate material along each path byapplication of a predetermined flux per unit area; repeating therelative movement of the substrate material and laser beam along eachpath for a number of passes until the material has been ablatedsufficiently to form a series of grooves of predetermined depth; and thenumber of passes of the laser beam along each groove being in the rangefrom 20 to
 120. 12. The method as claimed in claim 11, wherein the focusof the laser beam is adjusted by 2 to 6 microns after each 20 passes ofthe laser beam.
 13. The method as claimed in claim 1, wherein the laserbeam has a Q-switched pulsed output, and the first pulse of the laseroutput is eliminated prior to application of the laser output to thesubstrate.
 14. The method as claimed in claim 1, including the step ofapplying a coating layer of metal to the first surface of the substrateprior to application of the laser beam, whereby the metal layer as wellas the substrate is ablated along the groove.
 15. The method as claimedin claim 1, including the step of controlling operation of the laserbeam in conjunction with the relative movement of the substrate andlaser beam such that the laser beam is actuated only when the relativemovement is at a constant velocity.
 16. The method as claimed in claim1, wherein the substrate comprises an optically transmitting materialhaving opposite surfaces and a layer of phosphor material bonded to oneface of the optically transmitting material.
 17. The method as claimedin claim 7, including the steps of directing the laser beam at a firstsurface of a second substrate to be micro-milled, moving the secondsubstrate relative to the laser beam at a predetermined constantvelocity along a predetermined x-y grid pattern so as to ablate thesurface of the second substrate by application of a second predeterminedflux per unit area, and repeating the relative movement of the substrateand laser beam to form a series of pixels with grooves separating thepixels, and bonding the first surfaces of the two substrates togetherwith their pixels in alignment, the first substrate comprising aphosphor material bonded to a support and the second substratecomprising a light transmitting material.
 18. The method as claimed inclaim 1, wherein the laser beam and the number of passes along thegroove are controlled to form a groove having a width in the range from4 microns to 15 microns and a depth of approximately 1 mm.
 19. Themethod as claimed in claim 1, wherein the laser beam, velocity, andnumber of passes along the groove are controlled to form a groove havingan aspect ratio in the range from 5:1 to 150:1.